Raster input scanners typically comprise one or more linear arrays of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors. In a full-color raster input scanner, there are typically provided three linear arrays, each array has associated therewith a translucent primary-color (such as RGB) filter; the signals from each primary-color-filtered array can subsequently be combined as color separations of a full-color image. For high-performance image sensor arrays, a preferred design includes arrays of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging generally without the use of reductive optics. In one known design, an array is intended to be made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors, spaced at 400 photosensors per inch, per linear array.
In the context of a digital copier, optical conditions relating to the original input scanning of the original can result in print quality shortcomings and artifacts in, for example, a color copy made from the originally-scanned image data. Many of these possibly-detrimental optical conditions are closely related to the basic photosensor architecture, or photosensor configuration, of the raster input scanner. Since the human eye cannot detect high spatial frequency changes in chroma (color), but can detect high frequency changes in luminance (dark to light), it may be desirable to have photosensors that have higher resolution luminance detection and lower resolution chroma detection. This is especially true if one is trying to minimize the data bandwidth for a given level of human visual image quality. Another consideration is having signals from all detecting photosensitive regions that are about the same level so that the signal to noise ratio (SNR) is comparable for all color separation signals.
In addition, the translucent filter layers placed on some or all of the photosensitive regions of a photosensor array will change the signal level for a given light of an original color. It is also not desirable to sub-sample color as in Bayer's pattern, because of aliasing or Moire effects. All of the above goals conflict to some extent, if commonly used methods are used for the layout and readout of the multi-resolution device.